This invention relates generally to a liquid crystal display device, and more particularly, to a liquid crystal display device having a reflector and which provides a display with improved brightness.
Super twisted nematic (STN) liquid crystal display devices including a uniaxial anisotropic member (an oriented high molecular weight polymer sheet) between the polarizers of the device have been proposed to improve the display contrast. An example of such a liquid crystal display device 10 with a uniaxial optically anisotropic member 12 is shown in FIG. 1. Device 10 includes a twisted nematic liquid crystal display cell 13 with an upper linear polarizer 11 and a lower polarizer 14 on the outer surfaces of cell 13 and optically anisotropic layer 12. Display cell 13 includes an upper substrate 15 and lower substrate 16 with transparent electrodes 17 and 18 disposed on the inner surfaces and a twisted nematic liquid crystal material 19 therebetween. A spacer 22 holds substrates 15 and 16 apart and liquid crystal material 19 therebetween. Liquid crystal material 19 is twist oriented by rubbing the interior surfaces of substrates 15 and 16. Device 10 would normally be back lit.
FIG. 2 shows the relationship between the axes of these elements. In FIG. 2, R15 and R16 designate the rubbing directions of upper substrate 15 and lower substrate 16. Angle T2 designates the direction and angle of twist of the liquid crystal molecules in material 19 from upper substrate 15 to lower substrate 16. P11 and P14 designate the directions of the axes of polarization of upper polarizer 11 and lower polarizer 14, respectfully. A line A--A' identifies the direction of observation of device 10. .THETA..sub.45 is the angle between direction of observation A--A' and rubbing direction R15 of upper substrate 15 and .THETA..sub.46 is the angle between direction of observation A--A' and rubbing direction R16 of lower substrate 16. .THETA..sub.45 and .THETA..sub.46 are approximately equal to each other. Device 10 is described in detail in applicant's U.S. Pat. No. 4,844,569, the contents of which are incorporated herein by reference.
In device 10, the retardation value of optically anisotropic member 12 and the relationship between the axes are set for use in an STN display mode in order to improve the contrast and allow for a black-and-white display. While this arrangement has improved contrast and achieved nearly a full black and white display compared to ordinary STN devices, viewing angle characteristics were not considered.
FIG. 11 is an alternative embodiment of a STN liquid crystal device including a phase difference plate between two polarizing plates to improve the contrast of the liquid crystal device, but including a reflector, rather than being intended to be back lit.
In a preferred embodiment of the instant invention, the STN liquid crystal material 145 of FIG. 11 is selected so that the liquid crystal molecules are homogeneously aligned so as to provide a helical structure having a twist angle of 180.degree. to 270.degree. and so that the layer of STN liquid crystal material has positive dielectric anisotropy. In FIG. 11, the STN liquid crystal material 145 is sealed between the gap defined by glass substrates 143 and 147. Upper and lower polarizing plates 141 and 148 are respectively disposed on the upper surface of the phase difference plate 142 and the lower surface of the lower glass substrate 147. Phase difference plate 142 is an optically anisotropic member. In addition reflector 149, which is formed of a high reflectance material, for example aluminum, is provided on the lower surface of lower polarizing plate 148.
In the past, because these phase difference plates are costly, the most efficient method for cutting them from stock is usually used. This most efficient cutting method is to have the direction of the optical axis of the phase difference plate to be either coincident with or perpendicular to the direction in which the device is normally viewed (observation direction) when the plate is cut from stock. As used herein, the observation direction is a direction in the plane of the display screen defined by the liquid crystal display cell formed by the substrates and liquid crystal material. The rubbing directions and the directions of the absorption axes of the set of polarizing plates are then conventionally determined by taking into consideration the colors and contrasts.
The conventional liquid crystal display devices which, utilize a phase difference plate having the optical axis coincident with or perpendicular to the direction in which the device is normally viewed, and a reflector, have an unfavorably low display brightness.
Accordingly, it is desirable to provide a liquid crystal display device with a reflector which has increased brightness by choosing the direction of the optical axis of the phase difference plate.